They asked male and female students to look at 21 human silhouettes, all of them the same height, but with a progressively changing waist-to-hip ratio.

The figures began with an obviously female "hourglass" figure and, after incremental changes, ended with an obviously male "hunk" figure.

The volunteers were asked to say whether each of the 21 silhouettes was male or female, the idea being to identify the point where they saw a shift in gender.

What was striking, says researcher Kerri Johnson, was a preference for the volunteers to deem a shape to be a man whenever it was ambiguous - or could readily have been taken for a woman.

"I was surprised by the size of the effect. It was a much stronger effect than I ever imagined," says Johnson.

In the natural world, the demarcation between a woman's shape and man's shape comes when the ratio of the waist and hip circumferences is 0.8.

But the volunteers, on average, placed the boundary at 0.68. In other words, an identifiable female shape for them was close to the idealised curves of a pinup.

Johnson's team carried out three further studies, using a slightly different methods to see whether their approach had been skewed, and found that the bias in favour of men was unchanged.

Ancestral survival mechanism

Are these errors in perception?

Not so, says Johnson, who believes it to be an ancestral survival mechanism.

A man is likelier than a woman to be a bigger physical threat and our default perception is to prepare for risk: it's better to be safe than sorry.

"We suspect that this might be for a self-protective reason," she says.

"If you are walking down a dark alley at night, a woman poses no great physical threat to you in general, but if you encounter an unknown man, he's more likely to have a physical formidability that could pose some risks."

Johnson concedes that there could be cultural or ethnic factors which influence judgement but argued that the same kind of bias would prevail anywhere.

"I think it's entirely likely that if we were to test this in different populations we would probably have the same basic effect, the same pattern of judgement, although the strength of the judgement might vary," she says.

The findings show how gender stereotypes can be reinforced, sometimes dangerously so, according the researchers.

A woman could struggle if she has a body shape that is perceived as masculine and thus unattractive.

"Consistent with other research, this is likely to produce preferences for extreme body shapes, particularly for women," they write.